Dust control system

ABSTRACT

A road cleaning or sweeping machine ( 10 ) fitted with a dust control system using a water atomiser ( 42, 60 ). The atomiser ( 42, 60 ) creates water particles of a similar size to dust, and by entraining atomised water through the road sweeping machine&#39;s dust handling systems, i.e. from the brushes ( 14 ), through a suction pipe ( 20 ) and into a hopper ( 16 ), dust control is established. A method of improving dust separation in a hopper ( 16 ) of a road sweeping machine ( 10 ) using a water atomiser to reduce dust or a spray to wet the walls of the hopper ( 16 ).

The present invention relates to a dust control system, and in particular a road cleaning machine, or a road sweeping machine, fitted with the dust control system. The terms “road cleaning” and “road sweeping” are used broadly to include cleaning and sweeping of other areas such as pedestrian precincts, footpaths, car parks etc.

Road cleaning machines are machines for lifting dirt off the road and into a hopper. Road sweeping machines are road cleaning machines that are designed to brush the dirt off the road, generally towards, or into, a hopper. In many road sweeping machines, a suction pipe (or hose) along with a ground following suction nozzle is provided for sucking swept dirt from the road, and to act as a conduit for passing that sucked up dirt into the hopper. A road cleaning machine can, however, just comprise the suction pipe and the hopper, i.e. no sweeping mechanism.

The hopper is usually fitted at or on the back of the road cleaning or road sweeping machine. The pipe is usually connected to a vacuum source for providing the suction (e.g. a fan). The dirt may pass through the vacuum source (e.g. a fan designed to allow the passage of dirt and debris through it) or it may bypass the vacuum source (e.g. a clean fan).

The brushes for brushing the road are usually formed either from metal wires or from stiff polymeric fibres (which may be about 2 mm in diameter and 10 to 15 cm long for a compact road sweeping machine). The brushes, often two counter-rotating brushes, and sometimes more than two, are usually mounted on the road sweeping machine for rotation about a generally vertical axis. In a machine with two counter-rotating brushes, the brushes move the dirt towards the centreline of the road sweeping machine for lining it up for the suction pipe. The front nozzle of the suction pipe, therefore, is usually fitted on that centreline, behind the brushes.

As a result of the sweeping action, the dirt on the road can be moved a considerable distance by the brushes. When the road is dry, the dirt will also usually be dry. Therefore, the action of the brushes moving the dirt will often tend to create a significant amount of airborne dust, unless some form of dust control is provided. This dust, therefore, is a first form of dust that needs to be controlled.

In road cleaning machines that have a suction pipe, the dirt and dust is also moved a significant distance through the pipe, or conduit, and into the hopper. This also stirs up the dirt and creates airborne dust. Yet further, the hopper may be designed to swirl the dirt and air within the hopper with a cyclone effect to separate the dirt from the air. This also stirs up airborne dust in the hopper.

The hopper is usually vented to the atmosphere to allow the build up of pressure in the hopper, caused by the sucking of air and dirt into the hopper, to be dissipated. As a result of the stirred up dirt and dust within the hopper, the hopper will inevitably exhaust dust through the vents provided for this purpose, again unless some form of dust control for the suction pipe or hopper is provided. This is therefore another form of dust that needs to be controlled.

In prior art road sweeping machines, to control or reduce these two dust sources, it is conventional to provide some form of dust control:

1. With respect to the dust generated by the brushes, water sprays or sprinklers are commonly used by road sweeping machines (or road sweeping equipment) either to knock down or control the dust before it is created. For example, most sweeping machine manufacturers use or fit water sprays or sprinklers ahead of their brushes. This firstly wets the dirt before the brush touches it, thereby substantially reducing the amount of airborne dust created upon sweeping the dirt. Secondly, the water helps to wet airborne dust, if the brushes create any, to knock it down.

2. Manufacturers of road sweeping machines also introduce water spray into the suction pipe. This helps to reduce the amount of airborne dust entering the hopper, although dust will still be generated in the hopper by the swirling air in the hopper.

3. Truck mounted sweepers, i.e. large road sweeping machines, also spray water for dust control. In particular they spray water ahead of the nozzle of the suction pipe, but behind the brushes, since the larger size of the machine provides plenty of room for doing so. They also spray water within the suction pipe between the nozzle and the hopper, as described above, and in front of and perhaps onto the brushes.

In all these prior art road sweeping machines, however, the sprays or sprinklers used for dust control or dust suppression use coarse spray jets. These attempt to knock down the dust or to wet the dirt to minimise dust creation. However, coarse jets require large amounts of water (indeed, the more water the better). As a result, however, the road sweeping machines have to carry a large amount of water. Further, the road sweeping machines will suffer from the added cost of needing to be designed to handle or carry the large quantities of water that will be sucked back up into in the hopper. Therefore, a big, sturdy machine is required. As a result of the increased weight of the machine, more fuel will be required to power the machine. Yet further, as it has been wetted, the dirt has a high water content. Therefore, a lower percentage of sweepings (as opposed to sweepings and water) Will be collected per load in the hopper as dust control water will take up a high percentage of the load.

To help prevent the inevitable dust in the hopper from exiting the hopper, a filter of some form is also usually provided for the air vents. However, the dust will often be finer than the filter, so this solution is not totally effective.

A more effective or efficient dust control system is therefore desired for road cleaning or road sweeping machines.

The present invention provides a road cleaning machine comprising a suction pipe and a hopper for collecting dirt sucked up by the suction pipe, and a dust control system comprising a water atomiser arranged to dispense an atomised spray of water droplets into the air in front of the suction pipe so as to be entrained in dust-laden air being sucked into the suction pipe.

The invention also provides a method of improving the efficiency of a cyclone-type air/dirt separation system in a road cleaning machine, comprising wetting the internal walls of the cyclone unit so that dirt or dust sticks to the water on the walls.

The water atomiser creates finely atomised water. This is much more efficient at dust knock down than the prior art water sprayers.

When water particles are approximately as small as, or more preferably at least as small as, the dust particles, dust “knock down” can be so effective in a dust control system that wetting the dirt on the road is no longer essential for controlling dust creation by the brushes, or for controlling the dust exhaust from the hopper. Water atomisers create such small water particles. However, water sprayers and sprinklers, as found on prior art road cleaning machines, do not. The water particles from the water sprayers and sprinklers from the art of road sweeping machines, simply, are too large to control fine airborne dust effectively. As a result, in the water sprayers and sprinklers of prior art road sweeping machines, large quantities of water are needed in order for the dust control system to be as effective at dust control as the dust atomiser used in the. dust control system of the present invention.

The atomised droplets from the water atomiser preferably include a majority of droplets having a diameter of less than 200 μm, more preferably less than 100 μm and most preferably less than 40 μm and possibly less than 20 μm. Preferably the average diameter of the droplets is between 70 μm and 40 μm. Most preferably the average diameter is about 40 μm. Such droplets effectively float in the air (like a cloud). As a result, they are easily entrained into the suction pipe's airflow. Yet further, they are readily distributable throughout the airflow so that the water droplets can better contact the dust particles. This therefore controls the dust in the airflow of the suction pipe (or conduit), and also in the hopper, very effectively.

Atomising the water is also an efficient use of water. A large amount of “cloud” can be created with a small amount of liquid water. Therefore the water tank volume can be kept to a minimum, thus maximising the volume of space on the machine available for the rubbish hopper. This maximises the rubbish handling capacity of the machine—a larger hopper means more rubbish can be handled. This advantage is maximised when no other water-using, road cleaning/sweeping/washing devices are fitted to the road cleaning machine since a very small water tank can be fitted.

Preferably, for a road cleaning machine having a hopper designed to carry 1000 litres of rubbish, the dust control system of the road cleaning machine is provided with a water tank having a volume of about 200 litres. However, smaller tanks may also be provided, for example one of about 150 litres, or about 100 litres. Therefore, preferably the tank volume is about 20% of the rubbish volume capacity of the hopper, or 15% of the volume capacity or even 10% of the volume capacity. If two or more tanks are provided, the above figures relate to the total volume of the tanks.

Due to the low mass density of the normally collected debris (e.g. empty drink cans or bottles), the typical load that will be carried in a 1000 litre capacity hopper, when full, is about 500 kg. With the preferred water tank volume of 200 litres, the water in it will have a maximum mass of about 200 kg. In the event that all of the atomised water is sucked into the hopper, this volume of water will not overload the hopper. To ensure this is the case, preferably the water tank capacity in kg (i.e. the volume in litres) is no more than 40% of the weight capacity of the hopper in kg.

Preferably the road cleaning machine is a road sweeping machine. Preferably two counter 15 rotating brushes are fitted to the front of the road sweeping machine. More brushes may be provided. Each brush, or the road sweeping machine generally, may be fitted with a conventional water sprayer or sprinkler for wetting the dirt in front of the brushes. These dust control systems would be in addition to the water atomiser.

Preferably a hopper is provided at the back of the road sweeping machine.

Preferably the suction pipe is in a fixed position on the machine.

Preferably the atomiser is fitted above or in front of the entrance of the nozzle of the suction pipe. It has been found that maximum dust control is achieved by entraining the atomised water stream into the airflow at the entrance to the nozzle. The atomiser is preferably placed 0.1 m to 0.3 m above and/or in front of the entrance to the nozzle. This not only works effectively, but also has the further advantage that the atomising device does not get hit by debris in the suction airflow. It will also then be well clear of the ground, thus avoiding contact with objects on the ground. A third advantage is that the airflow at that point is relatively slow. Therefore, the cloud created by the atomiser will have time to be attracted to dirt in the airflow before it is sucked into the nozzle.

Preferably the water atomiser is made to be retro-fitted onto existing road cleaning or road sweeping machines, preferably between the brushes and suction pipe of a road sweeping machine, or at the exit of the suction pipe, e.g. in the hopper.

Preferably the atomiser is positioned well in front of any significant airflow constriction. For example a suitable distance might be 10 cm or more in the low speed airflow environment outside the suction pipe. Most road sweeping machines, however, will not have such airflow constrictions, other than the suction pipe or fan. The space is provided to give the cloud sufficient dwell time to penetrate dirt entrained in an airflow passing through it.

It has been noted that a cloud will tend to agglomerate back into coarse droplets if it is squeezed through a narrow orifice. However, once droplets have wet the dirt, this matters less, so the positioning in front of the constriction is to provide time for the cloud to wet the dirt. Further, by positioning the atomiser in front of the suction pipe, the cloud of water is intense in the nozzle, thereby mixing well with the dirt as it is lifted off the ground by the suction pipe or swirled-up by the brushes.

The atomiser could be positioned to the side of the nozzle. However, above it is more preferable since from the side it might not mix with the dust as efficiently, and the atomiser might hit objects on the ground as the machine drives along.

Preferably the atomiser is in a position that is both in front of the nozzle and next to the brushes but preferably physically out of the dirt's path to avoid it from getting damaged. As a result, dust that is kicked up by the brushes is also contained or controlled by the misted air (i.e. the atomised spray). Further, by positioning it there, a mixing of airborne dust and the atomised water will start prior to the dust entering the suction system.

In another aspect the invention provides a road cleaning machine comprising a suction pipe and a hopper for collecting dust sucked up by the suction pipe, and a dust control system comprising an atomising water spray within the hopper to control dust within the hopper.

Preferably the water atomiser comprises a rotary sintered spray head. Such a head produces finely atomised water. A preferred atomiser is a Newland Design (RTM) rotary 10 atomiser. It has a fast spinning motor, preferably spinning at up to 20,000 rpm. The Newland device is attractive because it is low energy and is not particularly sensitive to water cleanliness, thereby minimising servicing costs due to failing atomisers.

Another preferred water atomiser is a high pressure fine nozzle, water jet.

The particle sizes from either of these systems will generally be of an average size of about 40 μm.

The water fed to the atomiser is preferably substantially clean water from a water tank on the road cleaning machine, which water tank is fillable by the operator. The water, however, may be treated, for example with a soap, an antibacterial agent or a detergent. Preferably the treatment is a non-foaming treatment.

Such treatments generally will lower the surface tension of the water for the atomiser. As a result, smaller water droplets will more readily be created by the atomiser.

Preferably the particle size is also controllable. This may be by varying either the pressure or the speed of the rotor. For example, at a rotor speed of about 20,000 rpm, the Newland rotary atomiser produces water droplets having a size of about 40 μm, whereas increasing the speed, perhaps up to 30,000 rpm, makes the droplets smaller, and reducing the speed makes the droplets bigger. This rotor speed can be made to be adjustable by a dial or control operable by the operator from within the cab of the road sweeping machine, or perhaps automatically by the machine. This adjustability is useful for the following reasons:

It has been noted that in windy conditions the atomised water will tend to be blown about. However, whereas very fine particles are blown a great deal in a wind, slightly larger particles are blown a lesser amount. By adjusting the speed of the rotor, for example, the droplet size can be varied. Therefore an operator can adjust the particle size to be larger if the conditions are very windy, or smaller when conditions are relatively still. A wind sensor can be fitted to the vehicle to determine wind speeds in order to allow the machine to do this automatically. Alternatively, the machine may be adapted to sense the amount of atomised water being blown away in some other manner and automatically adjusting the droplet size accordingly.

The atomiser may be positioned in the hopper to control dust in the hopper, either at the entrance of the hopper, preferably at the exit of the suction pipe, or somewhere else in the hopper.

An atomiser, or the atomiser, may be positioned in front of the brushes further to control dust from the sweeping action.

More than one atomiser may be provided in each or any position, thereby allowing atomisers to be placed in more than one place, and/or to increase the cloud density at any one place.

The road cleaning or sweeping machine may additionally have conventional dust control systems, such as known water sprays or sprinklers, such as in front of the brushes or in the suction pipe. These would help control dust for example where the dirt is particularly prone to dust creation, such as on very dusty or dirty, and dry, roads.

Where water-using, conventional, dust control systems are provided, larger water tanks may be provided for the machine since a lower percentage of the used water will be collected up off the road by the suction pipe. Therefore, for a road sweeping machine having a hopper capacity of 1000 litres. A water tank having a volume of between 150 and 400 litres would be desirable (i.e. between 15 and 40% of the hopper capacity by volume). A water tank having an approximate volume of 200 litres would be most preferable i.e. about 20% of the hopper capacity by volume.

The or each atomiser may be positioned substantially above one or more of the brushes.

The or each atomiser may be substantially between two or more brushes.

The cloud from the or each atomiser will preferably be targeted or directed in order better to suppress or control dust generation from the brushes.

Preferably the cloud will suppress or control dust generation from one or more of the following dust sources: the brushes, the machine's wheels, any atmospheric wind passing through the machine, or the wind generated by the motion of the machine (which may be being driven at 10 miles an hour), and the suction pipe or swirling of air (and entrained dirt) in the hopper.

The atomiser may also be advantageous when used in regenerative air sweepers on these machines, such as where partially cleaned air in the hopper is sucked out of the hopper, returned to a large suction head, transversed across the suction head picking up debris and which is then sucked back into the hopper.

Preferably the dust control system is switchable between an on and an off setting for conserving water when dust control is not required. Preferably the dust control system is automatically turned off when the machine is not sweeping or cleaning, e.g. during transport to or from the depot.

In a preferred machine, the machine comprises a dust sensor and the dust control system is adapted to be switched on automatically upon sensing dust at the sensor. The sensor may be behind the brushes, in the suction pipe, in the hopper, or at an outlet air vent of the hopper.

The dust sensor is preferably at the or each outlet air vent of the hopper so that the dust control system would only be switched on when dust was being vented from the hopper (or when dust has recently been sensed as being vented). This conserves water as well. This is particularly useful since the air vents usually are not visible to the operator of the road cleaning machine when he is using the machine.

Preferably the machine comprises an automatic control for the atomiser, using the machine's computer or electronic control system for controlling the atomiser. The control may be adapted so as to have the water supply shut off, e.g. by a solenoid valve, prior to stopping the high speed motor of the suction system to prevent water from being allowed to remain inside the suction system when it is not in use, which water could damage the rotor.

Preferably the vacuum source for the suction pipe is a powerful fan. Preferably the fan is a clean air fan. Preferably this is achieved by placing the fan at the downstream end of the airflow through the road cleaning machine, preferably just prior to, i.e. upstream of, the outlet vent for the machine, rather than by using a bypass system. By positioning it here, although the sucked up air passes through the fan, the dirt will have been separated out of that air by there, preferably within the hopper. Preferably the hopper achieves this by virtue of a cyclone action, and more preferably by virtue of a dual cyclone action, the first cyclone action being in the main body of the hopper and the second cyclone action being in a dedicated cyclone unit positioned further down stream of the main body of the hopper. The powerful fan is required to drive these two cyclones.

Preferably the suction pipe passes the sucked up air and dirt into the hopper near a side of the hopper. Preferably the sucked up air and dirt is passed into the hopper in a direction that does not point directly or generally towards the vertical centreline of the hopper, which direction is preferably substantially parallel to a side wall of the hopper, or substantially internally tangential in the case of a vertical, cylindrical, hopper. The inflow is also preferably not horizontal, more preferably being pointed slightly upward. By directing the air inflow in this manner, a vertically oriented swirling of the air and dirt in the hopper is induced. This sets up a cyclone effect in the main body of the hopper.

Preferably the air and dirt from the suction pipe enters the hopper about midway up the front wall of the hopper, but generally at the side thereof to create the swirling effect of the cyclone.

Preferably the generally vertical corners the hopper are rounded internally by swirl plates, to enhance the cyclone effect further.

Preferably a coarse water spray is provided at the exit of the suction pipe or within the hopper, preferably by a water spray jet. By spraying such water there, the water will tend to wet the sidewalls of the hopper to provide a film of water on the inner surface of the hopper wall, and, by virtue of the cyclone effect, the roof of the hopper as well. Dust or dirt entrained in the air passing out of the suction pipe will hits the walls, thereby being wetted further by the water on the walls and roof. The dirt will become a mud or a sludge and will tend to gather at the walls, thereby being extracted from the airflow. The resultant mud or sludge will then either trickle down the wall, or fall off the wall or roof, so as to settle at the bottom of the hopper.

This feature of the present invention enhances the separation method even further and provides a more compacted litter in the bottom of the hopper than conventional, top-centre fed hoppers. Indeed, it has been found that with this system, in normal road sweeping conditions, a 1000 litre hopper will collect a similar amount of dirt as can be collected as in a 1600 litre top-centre fed hopper.

The coarse water spray can also be fitted elsewhere in the airflow for wetting the walls or roof to maximise this effect.

If the coarse water spray is positioned further upstream from that preferred position, e.g. in the suction pipe, it has been noted that dirt will tend to accumulate Just downstream of the spray, causing a blockage prior to the hopper. This blockage would need to be cleared, so that position for the coarse water spray is undesirable. Such blockages in the hopper, however, are not a problem—a blockage in the hopper cannot occur unless the blockage is due to the hopper being full.

The present invention also provides a retro-fit kit for fitting to a road cleaning machine comprising machine, comprising a water atomiser and mounting means for fitting the water atomiser to the machine so as to dispense an atomised spray of water droplets into the air in front of a suction pipe of the machine so as to be entrained into dust-laden air being sucked into the suction pipe. The kit may also comprise any of the features of the dust control system of the road cleaning machine described above.

The invention further provides a retro-fit kit for a road cleaning machine, comprising a water atomiser and mounting means for fitting the water atomiser to the machine in the hopper thereof.

The present invention also provides a method of improving the efficiency of a cyclone-type air/dirt separation system in a road sweeping machine, comprising providing a water atomiser in the system and atomising water therewith into the path of the air and dirt. The cyclone may be formed within the main body of the hopper or in a cyclone unit either provided within or fluidly attached to the hopper. The water may be atomised into the path either in the hopper, or outside the hopper, as described above for the dust control system.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic front view of a road sweeping machine in accordance with the present invention;

FIG. 2 is a schematic side view of the sweeping machine of FIG. 1;

FIG. 3 is a schematic plan view of the road sweeping machine of FIG. 1; and

FIG. 4 is a schematic plan view of the road sweeping machine showing the water spray jet for applying a film of water onto the inner surface of the hopper wall.

The present invention provides a road cleaning or sweeping machine (10) fitted with a dust control system using a water atomiser (42, 60). The atomiser (42, 60) is a dedicated unit for creating water droplets of a similar size to dust, and by entraining atomised water through the road sweeping machine's dust handling systems, i.e. from the brushes (14), through a suction pipe (20) and into a hopper (16), dust control is established.

The illustrated road sweeping machine (10) comprises a cab (12), four wheels (13)—two shown, two counter-rotating brushes (14), a hopper (16) (see FIGS. 2 and 3) and a suction pipe (20). The cab (12) is located at the front of the machine (10). The two counter-rotating brushes (14) are positioned underneath and slightly in front of the cab (12) for sweeping dirt off the ground (18). The hopper (16) is for the collection of the dirt and debris picked up from the ground (18). The suction pipe (20) is driven by a fan (21) (see FIG. 2) for 5 sucking up the dirt from the ground (18) for passing it from the ground (18) into the hopper (16).

The suction pipe (20) has a suction nozzle (22) that has a rearmost skirt (24) (see FIG. 2), side skirts (26) (see FIG. 1) and a roof component (28) (see FIG. 2). The rearmost skirt (24), the side skirt (26) and the roof component (28) form an open-fronted and open-based suction nozzle through which dirt and rubbish can easily be sucked up from the ground (18) into the suction pipe (20) for passage through the conduit of the suction pipe (20) into the hopper (16). The fan (21) is shown to be a clean air fan (21) in the sense that the dirt and debris collected in the hopper (16) does not pass through the fan (21) on its way to the hopper (16). A dirty fan, however, could instead be provided, i.e. one through which the dirt and debris is passed on its way to the hopper (16).

The fan (21) is positioned downstream of the hopper (16), near the air outlet (40) of the road sweeping machine (10). It sucks the air and dirt all the way through the road sweeping 20 machine (10), the dirt and dust being separated from the air on its way through the hopper (16). The fan (21) does, however, provide the final blowing of the hopper exhaust out through the air outlet (40).

Dirt and air is sucked up into the suction pipe (20) via the nozzle (22), and is then passed 25 into the hopper (16) through the suction pipe (20). It exits the suction pipe (20) through the suction pipe's exit (30), which exit (30) is within the hopper (16). The flow of air from that exit (30) drives an initial cyclonic action (32) within the main volume of the hopper (16) due to the exit being substantially at a side of the front of the hopper, and due to the high power of the fan, whereby heavy solids (34) are separated from the air and the lighter 30 debris. The lighter debris and air will then eventually pass through a filter screen (36) to separate further light debris from the air. However, fine particulate matter will pass through that screen (36). That air and fine particulate matter will then enter into a secondary cyclone unit (38) for separation of additional matter from the air.

A water atomiser (42) is provided in front of the suction pipe (20), between the two brushes (14). As a result, dirt and dust passing through the hopper is wetted. This makes the dust heavier. Therefore, the cyclone systems are made more efficient. As a result, the air passing out of the secondary cyclone unit will be substantially clean and therefore it is passed out to atmosphere through the fan (21) via the air outlet (40). However, that air, or part of it, could alternatively be recirculated back through the hopper cyclone system after the fan (21) by providing a loop for the air to a location in front of the nozzle, whereby it will be sucked back into the hopper (16), sucking up more dirt on the way. This feature, however, is not shown in the drawings.

The screen (36), which separates some of the light matter from the air is provided in the roof of the hopper (16). The roof also serves as the hopper's door (44) (when emptying the hopper (16), the hopper (16) is rotated up and out of the machine (10) about a hinge (46). Accordingly, the hopper door (40) will then be correctly positioned to allow efficient emptying of the hopper (16).

The water atomiser (42), as mentioned above, wets the dirt or the dust particles. This not only makes the two cyclones more effective, but also minimises the airborne dust creation as a result of the counter-rotating brushes and the air movement within the sweeping machine, since much of the dirt will be picked up in lumps.

Within the hopper (16), and in particular in the comers of the hopper, swirl plates (50) are provided. This enhances the initial cyclonic action, by keeping a circular motion of the air and minimising eddies.

As shown in FIGS. 1 and 2, the atomiser (42) is provided just in front of, and above, the roof component (28) of the suction nozzle (22). This position is also substantially between the brushes (14). In this position, the water atomiser, which is fed water via a hose (not 30 shown) from a water tank (not shown), creates an atomised cloud (52) that exists both between the brushes (14) and in the suction nozzle (22) by virtue of the suction from the suction pipe (20).

The atomised cloud (52) has a majority of water droplets in it having a size or diameter of less than about 200 μm, and preferably less than about 100 um, or most preferably of approximately 40 μm. Typically the water particles have an average diameter of between 40 μm and 70 μm.

The water atomiser is a Newland rotary atomiser, which has a fast spinning motor, preferably spinning at speeds of up to 20,000 rpm, but which may be spun even faster, if desired to reduce the droplet size, or slower for larger droplet sizes, e.g. in windy conditions. These types of atomiser are commercially available for use in other arts, such as for optimising pesticide use. Accordingly, the constructional details of the atomiser need not be discussed further herein. It will be appreciated, however, that other atomisers could be used with the present invention.

Until now, road sweeping machines have used high pressure water pressure pumps, typically of around 200 psi, for passing water through static jets. However, the water particles are relatively large water particles. Therefore, individually they will have only a limited effect on dust control. Nevertheless, with the use of a lot of water, they do create an effective dust control system.

Dust can range in size down to sub-micron particle size. The water atomiser used for the present invention creates water particulates in the water cloud of a size comparable with that of dust, i.e. within an approximate factor of 10. As a result of this smaller droplet size, the efficiency of dust knock down with the atomiser, as per the present invention, is significantly more efficient than the prior art water sprays and sprinklers.

Further, the atomised cloud will maintain its entrapment in the airflow through the road sweeping machine (10), whereas the prior art water spray would tend simply to fall to the ground, or to just wet the inside surface of the suction pipe (20), causing blockages, or to fall from the suction pipe (20), when entering the hopper (16), straight into the bottom of the hopper (16). As a result of the maintained entrapment in the airflow, as can be seen in FIG. 2, the atomised water (54) gets sucked through the suction pipe (20), and at this point it will already be mixed with dust and debris. The cloud, the dust and the debris (54) will then pass along the suction pipe (20) at high velocity and then enter into the hopper (16).

As shown in FIG. 3, atomised water cloud mixed with dust and debris (56) and gets entrained into the initial cyclonic action (32) within the hopper (16). The dust and dirt, if not already wetted, will therefore get wetted, and will become more heavy. As a result it will more readily fall out of the cyclone (32) as heavy solids (34).

Referring now to FIG. 4, a water spray jet (48) is also provided at the exit (30) of the suction pipe. It sprays water away from the wall into the airflow exiting the exit (30). The airflow carries that sprayed water, however, so that it wets the sidewalls, and possibly the roof of the hopper as a further result of the cyclone effect. The wetted walls and roof will then add to the effectiveness of the separation of the dust, the dirt and debris from the air since the resultant film (58) of water on the inner surface of the hopper will trap dirt. The sprayed water could be fed directly onto the walls eg. from a spray jet within the hopper, rather than using the airflow to distribute it, however.

After the main body of the hopper (16), the dust and the dirt, which is still entrained with the water cloud, would pass eventually to the screen (36) and, if not stopped by the screen, will pass through to the secondary cyclone (38). As the dust and air continues, it will continue to be wetted as it circulates through the secondary cyclone, thereby improving the efficiency of the secondary cyclone (38) as well.

After the secondary cyclone unit, the air should be substantially clean. It will pass along its final conduit, therefore, through to the fan (21), and then pass through the fan (21).

It would be possible to recycle the airflow back into the hopper (16) via the fan (21) and/or via the nozzle and suction pipe. This would yet further improve the efficiency of the dust/air separation system since firstly more wet air will be passed through the hopper (16) and secondly any dirt or dust left in the air being recycled would be reprocessed in the hopper. In the illustrated embodiment, however, the air just passed through the fan (21) and is then vented to atmosphere via the air outlet (40).

Referring again to FIG. 3, an alternative or supplemental position (60) for a water atomiser is shown—at the exit (30) of the suction pipe (20), i.e. within the hopper (16). This alternative or supplemental position (60) generates a further atomised cloud, specifically for within the hopper (16), thereby further enhancing the dirt separation within the hopper. This position would be suitable for the coarse water spray as well, for wetting the sidewalls of the hopper, again to enhance the dirt separation provided by the hopper (16).

The present invention has been described above purely by way of example. It should be noted, however, that modifications in detail may be made to the invention as defined in the claims appended hereto. For example, in a machine which does not have brushes, i.e. in a more basic road cleaning machine, the suction pipe may be used to suck up dirt from the ground without an atomiser in front of it. Instead, an atomiser (42, 60) may be provided within the hopper or along the suction pipe, or a coarse water spray at the exit (30) of the suction pipe (20), for enhanced dust control and dust separation. 

1-41. (canceled)
 42. A road cleaning machine comprising a suction pipe and a hopper for collecting dirt sucked up by the suction pipe, and a dust control system comprising a water atomiser arranged to dispense a cloud of atomised water droplets into the air in front of the suction pipe so as to be entrained with dust particles in dust-laden air being sucked into the suction pipe.
 43. The machine of claim 42, wherein water particles from the water atomiser are at least as small as the majority of the dust particles of the dirt being sucked up.
 44. The machine of claim 42, wherein the machine does not significantly wet the dirt on the road in front of the machine.
 45. The machine of claim 42, wherein the atomised droplets from the water atomiser include a majority of droplets having a diameter of less than 200 μm.
 46. The machine of claim 45, wherein the atomised droplets from the water atomiser include a majority of droplets having a diameter of less than 100 μm.
 47. The machine of claim 45, wherein the atomised droplets from the water atomiser have an average diameter of between 40 μm and 70 μm.
 48. The machine of claim 42, wherein the dust control system is provided with a water tank, or water tanks, having a volume of about 20% of the volume capacity of the hopper.
 49. The machine of claim 42 comprising one or more road-sweeping brushes.
 50. The machine of claim 49, wherein two counter rotating brushes are fitted to the front of the road sweeping machine.
 51. The machine of claim 50, wherein the atomiser is fitted so as to provide the cloud of droplets between the brushes.
 52. The machine of claim 42, wherein another water atomiser is positioned in the hopper.
 53. The machine of claim 42, wherein the machine recirculates air which has passed through the hopper back into the hopper.
 54. The machine of claim 53, wherein the recirculated air is directed to a location in front of the suction pipe for recirculating to the hopper.
 55. The machine of claim 42, wherein the or a said water atomiser comprises a rotary spray head.
 56. The machine of claims 42, wherein the or a said water atomiser is a nozzle.
 57. The machine of claim 42, wherein the average water droplet size from the atomiser is controllable.
 58. The machine of claim 42 comprising a dust sensor, the dust control system being adapted to be switched on automatically upon the sensor sensing dust.
 59. The machine of claim 42 comprising a water sprayer at the exit of the suction pipe or within the hopper for wetting the inside of the hopper.
 60. The road cleaning machine of claim 42, wherein the hopper comprises a cyclone separator.
 61. A retro-fit kit for a road cleaning machine, comprising a water atomiser and mounting means for fitting the water atomiser to the machine so as to dispense a cloud of atomised water droplets into the air in front of a suction pipe of the machine so as to be entrained with dust particles in dust-laden air being sucked into the suction pipe.
 62. A method of dust control in a road cleaning machine having a suction pipe and a hopper for collecting dirt sucked up by the suction pipe, the method comprising dispensing a cloud of atomised water droplets into the air in front of the suction pipe so as to be entrained with dust particles in dust-laden air being sucked into the suction pipe.
 63. The method of claim 62, wherein the atomised water is treated to reduce its surface tension.
 64. The method of claim 63, wherein the treatment is the addition of one or more of the following: a soap, a detergent or an antibacterial agent. 